Pulse tube refrigerator

ABSTRACT

In a pulse tube refrigerator, a first pulse tube has a low-temperature end, and a high-temperature end connected to a compressor. A first regenerator has a low-temperature end connected to the low-temperature end of the first pulse tube, and a high-temperature end connected to the compressor. A second pulse tube has a high-temperature end connected to the compressor, and a low-temperature end having a lower temperature than the low-temperature end of the first pulse tube. A second regenerator has a high-temperature end, and a low-temperature end connected to the low-temperature end of the second pulse tube, and is arranged side by side with the second pulse tube. The second pulse tube includes a narrowed portion nearer a low-temperature end side than a position corresponding to the high-temperature end of the second regenerator.

RELATED APPLICATION

Priority is claimed to Japanese Patent Application No. 2014-000398, filed Jan. 6, 2014, the entire content of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The invention relates to a pulse tube refrigerator, and particularly, to a multistage multi-valve type pulse tube refrigerator.

2. Description of Related Art

Pulse tube refrigerators are known as one of refrigerators that cause ultralow temperature. In pulse tube refrigerators, coldness is formed at low-temperature ends of a regenerator and a pulse tube by repeating an operation in which a refrigerant gas as a working fluid (for example, helium gas) compressed by a compressor flows into the regenerator and the pulse tube, and an operation in which the working fluid flows out of the pulse tube and the regenerator and is recovered in the compressor. Additionally, heat can be taken from a cooling target by bringing the cooling target into thermal contact with these low-temperature ends. Particularly, multistage multi-valve type pulse tube refrigerators have the feature of having high cooling efficiency, and applications thereof in various fields are expected.

SUMMARY

According to an embodiment of the present invention, there is provided a pulse tube refrigerator including a compressor that compresses a refrigerant gas; a first pulse tube that has a low-temperature end and a high-temperature end connected to the compressor; a first regenerator that has a low-temperature end connected to the low-temperature end of the first pulse tube and a high-temperature end connected to the compressor; a second pulse tube that has a high-temperature end connected to the compressor and a low-temperature end having a lower temperature than the low-temperature end of the first pulse tube; and a second regenerator that has a high-temperature end and a low-temperature end connected to the low-temperature end of the second pulse tube and is arranged side by side with the second pulse tube. The second pulse tube includes a narrowed portion nearer a low-temperature end side than a position corresponding to the high-temperature end of the second regenerator.

According to another embodiment of the invention, there is provided a pulse tube refrigerator including a compressor that compresses a refrigerant gas; a first pulse tube that has a low-temperature end and a high-temperature end connected to the compressor; a first regenerator that has a low-temperature end connected to the low-temperature end of the first pulse tube and a high-temperature end connected to the compressor; a second pulse tube that has a high-temperature end connected to the compressor and a low-temperature end having a lower temperature than the low-temperature end of the first pulse tube; and a second regenerator that has a high-temperature end and a low-temperature end connected to the low-temperature end of the second pulse tube and is arranged side by side with the second pulse tube . The second pulse tube includes a narrowed portion in a region where the temperature of the refrigerant gas flowing through the second pulse tube reaches 8 K to 30 K.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically illustrating the outline of an example of a four-valve type pulse tube refrigerator.

FIG. 2 is a view illustrating a series of open/closed states of six valves in the operation of the four-valve type pulse tube refrigerator illustrated in FIG. 1.

FIG. 3 is a view illustrating changes in the density of helium gas at 2.2 MPa and the density of helium gas at 0.8 MPa, with temperature and changes in the density difference between both the helium gases with temperature.

FIGS. 4A and 4B are views schematically illustrating the configuration of a second pulse tube related to an embodiment.

FIG. 5 is a view schematically illustrating the outline of an example of a four-valve type pulse tube refrigerator related to a modification example of the invention.

FIG. 6 is a view illustrating a series of open/closed states of eight valves in the operation of the four-valve type pulse tube refrigerator illustrated in FIG. 5.

DETAILED DESCRIPTION

Compressors compress low-pressure (for example, 0.8 MPa) helium gas, and generate high-pressure (for example, 2.2 MPa) helium gas. The density difference between the density of high-pressure helium gas and the density of low-pressure helium gas has a large temperature dependency near an ultralow temperature. As a result, particularly when a temperature is near 10 K, the density difference becomes a maximum. For this reason, when helium gas is used for a refrigerant gas of pulse tube refrigerators, the pressure difference of the refrigerant gas in the pulse tube refrigerators becomes small. As a result, since it is difficult to adjust the phase difference between the flow velocity and pressure fluctuation of the helium gas, this may become a factor that causes the refrigeration capacity of pulse tube refrigerators to decline.

It is desirable to provide a technique that improves the refrigeration capacity of a pulse tube refrigerator.

According to the invention, a technique that improves the refrigeration capacity of the pulse tube refrigerator can be provided.

An embodiment of the invention will be described together with the drawings.

A general four-valve type pulse tube refrigerator will first be described prior to the description of a pulse tube refrigerator related to the embodiment of the invention. FIG. 1 is a view schematically illustrating an outline of an example of a general four-valve type pulse tube refrigerator 200. The pulse tube refrigerator 200 has a two-stage structure.

As illustrated in FIG. 1, the pulse tube refrigerator 200 includes a compressor 212, a first regenerator 240 and a second regenerator 280, a first pulse tube 250 and a second pulse tube 290, first piping 256 and second piping 286, a first flow path resistor 260 constituted of an orifice, or the like, a second flow path resistor 261, and opening and closing valves V1 to V6, and the like.

The first regenerator 240 has a high-temperature end 242 and a low-temperature end 244, and the second regenerator 280 has a high-temperature end 244 (equivalent to the low-temperature end 244 of the first regenerator 240) and a low-temperature end 284. The first pulse tube 250 has a high-temperature end 252 and a low-temperature end 254, and the second pulse tube 290 has a high-temperature end 292 and a low-temperature end 294. Heat exchangers are respectively installed at the high-temperature ends 252 and 292 and the low-temperature ends 254 and 294 of the first pulse tube 250 and the second pulse tube 290. Since the low-temperature end 244 of the first regenerator 240 is the same as the high-temperature end 244 of the second regenerator 280, the first regenerator 240 and the second regenerator 280 are arranged so as to have a common longitudinal axis. Additionally, the first regenerator 240 and the first pulse tube 250 are arranged so that the longitudinal axes thereof are aligned side by side. The second regenerator 280 and the second pulse tube 290 are also arranged side by side so that the longitudinal axes thereof are aligned with each other.

The low-temperature end 244 of the first regenerator 240 is connected to the low-temperature end 254 of the first pulse tube 250 via the first piping 256. Additionally, the low-temperature end 284 of the second regenerator 280 is connected to the low-temperature end 294 of the second pulse tube 290 via the second piping 286. Accordingly, the temperature of a refrigerant gas at the low-temperature end 244 of the first regenerator 240 and the temperature of the refrigerant gas at the low-temperature end 254 of the first pulse tube 250 become substantially equal temperatures. Additionally, the temperature of the refrigerant gas at the low-temperature end 284 of the second regenerator 280 and the temperature of the low-temperature end 294 of the second pulse tube 290 also become substantially equal temperatures.

Since the low-temperature end 244 of the first regenerator 240 is the same as the high-temperature end 244 of the second regenerator 280, the low-temperature end 284 of the second regenerator 280 has a lower temperature than the low-temperature end 244 of the first regenerator 240. Therefore, the low-temperature end 294 of the second pulse tube 290 has a lower temperature than the low-temperature end 254 of the first pulse tube 250.

A refrigerant flow path on a high-pressure side (discharge side) of the compressor 212 branches into three directions at point A in FIG. 1, and a first refrigerant supply path H1, a second refrigerant supply path H2, and a third refrigerant supply path H3 are constructed. The first refrigerant supply path H1 is constituted of the high-pressure side of the compressor 212, first high-pressure piping 215A where the first opening and closing valve V1 is installed, common piping 220, and the first regenerator 240. The second refrigerant supply path H2 is constituted of the high-pressure side of the compressor 212, second high-pressure piping 225A to which the third opening and closing valve V3 is connected, common piping 230 where the first flow path resistor 260 is installed, and the first pulse tube 250. The third refrigerant supply path H3 is constituted of the high-pressure side of the compressor 212, third high-pressure piping 235A to which the fifth opening and closing valve V5 is connected, common piping 299 where the second flow path resistor 261 is installed, and the second pulse tube 290.

Meanwhile, a refrigerant flow path on a low-pressure side (suction side) of the compressor 212 branches into three directions of a first refrigerant recovery path L1, a second refrigerant recovery path L2, and a third refrigerant recovery path L3. The first refrigerant recovery path L1 is constituted of the first regenerator 240, the common piping 220, first low-pressure piping 215B to which the second opening and closing valve V2 is installed, a point B, and the path of the compressor 212. The second refrigerant recovery path L2 is constituted of the first pulse tube 250, the common piping 230 where the first flow path resistor 260 is installed, second low-pressure piping 225B where the fourth opening and closing valve V4 is installed, the point B, and the path of the compressor 212. The third refrigerant recovery path L3 is constituted of the second pulse tube 290, the common piping 299 where the second flow path resistor 261 is installed, third low-pressure piping 235B where the sixth opening and closing valve V6 is installed, the point B, and the path of the compressor 212.

Subsequently, the operation of the pulse tube refrigerator 200 will be described.

FIG. 2 is a view illustrating a series of open/closed states of the six opening and closing valves V1 to V6 in the operation of the four-valve type pulse tube refrigerator 200 illustrated in FIG. 1. Hereinafter, the view illustrated in FIG. 2 is also referred to as a “timing chart”.

As illustrated in FIG. 2, during the operation of the pulse tube refrigerator 200, the open/closed states of the six opening and closing valves V1 to V6 change periodically as follows.

First Process: Time 0 to Time t1

First, at time t=0, only the fifth opening and closing valve V5 is open. Accordingly, a high-pressure refrigerant gas is supplied from the compressor 212 via the third refrigerant supply path H3, that is, via the path of the third high-pressure piping 235A, the common piping 299, and the high-temperature end 292 to the second pulse tube 290. Thereafter, at time t=t1, the third opening and closing valve V3 is opened while the fifth opening and closing valve V5 remains open. Accordingly, the high-pressure refrigerant gas is supplied from the compressor 212 via the second refrigerant supply path H2, that is, via the path of the second high-pressure piping 225A, the common piping 230, and the high-temperature end 252 to the first pulse tube 250.

Second Process: Time t2 to Time t3

Next, at time t=t2, the first opening and closing valve V1 is opened in a state where the opening and closing valves V5 and V3 are open. Accordingly, the high-pressure refrigerant gas is introduced from the compressor 212, via the first refrigerant supply path H1, via the path of the first high-pressure piping 215A, the common piping 220, and the high-temperature end 242, into the first regenerator 240 and the second regenerator 280. A portion of the refrigerant gas flows from the low-temperature end 254 side via the first piping 256 into the first pulse tube 250. Additionally, another portion of the refrigerant gas passes through the second regenerator 280, and flows from the low-temperature end 294 side via the second piping 286 into the second pulse tube 290.

Third Process: Time t3 to Time t4

Next, at time t=t3, the third opening and closing valve V3 is closed while the first opening and closing valve V1 remains open. Thereafter, at time t=t4, the fifth opening and closing valve V5 is also closed. The refrigerant gas from the compressor 212 flows into the first regenerator 240 only via the first refrigerant supply path H1. Thereafter, the refrigerant gas flows from the low-temperature end 254 side and the low-temperature end 294 side, respectively, into the first pulse tube 250 and the second pulse tube 290.

Fourth Process: Time t4 to Time t5

At time t=t5, all the opening and closing valves V1 to V6 are closed. The refrigerant gas within the first pulse tube 250 and the second pulse tube 290 moves to a first reservoir 251 and a second reservoir 291, which are installed on the high-temperature end 252 and 292 sides of both the pulse tubes, due to the pressure rise of the first pulse tube 250 and the second pulse tube 290.

Fifth Process: Time t5 to Time t7

Thereafter, at time t=t5, the sixth opening and closing valve V6 is opened, and the refrigerant gas within the second pulse tube 290 returns to the compressor 212 through the third refrigerant recovery path L3. Thereafter, at time t=t6, the fourth opening and closing valve V4 is opened, and the refrigerant gas within the first pulse tube 250 returns to the compressor 212 through the second refrigerant recovery path L2. Accordingly, the pressure of the first pulse tube 250 and the pressure of the second pulse tube 290 drop.

Sixth Process: Time t7 to Time t8

Next, at time t=t7, the second opening and closing valve V2 is opened while the opening and closing valves V6 and V4 remain open . Accordingly, a major portion of the refrigerant gas within the first pulse tube 250, the second pulse tube 290, and the second regenerator 280 passes through the first regenerator 240, and returns to the compressor 212 via the first refrigerant recovery path L1.

Seventh Process: Time t8 to Time t10

Next, at time t=t8, the fourth opening and closing valve V4 is closed in a state where the second opening and closing valve V2 is open. Thereafter, at time t=t9, the sixth opening and closing valve V6 is also closed. Thereafter, at time t=t10, the second opening and closing valve V2 is closed, and one cycle is completed.

By adopting the above cycle as one cycle and repeating this cycle, coldness occurs at the low-temperature end 254 of the first pulse tube 250 and the low-temperature end 294 of the second pulse tube 290, and a cooling target can be cooled.

As described above, the pulse tube refrigerator 200 repeats the refrigerant gas, such as helium, being compressed by high pressure and expanding if the refrigerant gas is made to have a low pressure, thereby causing coldness. Here, the pressure of the high-pressure refrigerant gas supplied from the compressor 212 is about 2.2 MPa, and the pressure of the refrigerant gas at low pressure is about 0.8 MPa.

FIG. 3 is a view illustrating changes in the density of helium gas at 2.2 MPa and the density of helium gas at 0.8 MPa with temperature, and changes in the density difference between both the helium gases with temperature. As illustrated in FIG. 3, the density difference between helium gas at 2.2 MPa and helium gas at 0.8 MPa becomes a maximum when the temperature of the helium gases is about 8 K. When the temperature of the helium gases is lower than 8 K, the density difference between the helium gas at 2.2 MPa and the helium gas at 0.8 MPa monotonously increases with respect to temperature, and the density difference monotonously decreases with respect to temperature when the temperature of the helium gases is higher than 8 K.

In the pulse tube refrigerator 200, the temperature of the refrigerant gas at the low-temperature end 294 of the second pulse tube 290 is about 4 K. The refrigerant gas in the second pulse tube 290 has a temperature that is about room temperature at the high-temperature end 292. Therefore, the refrigerant gas in the second pulse tube 290 is present in a temperature gradient of about 4 K to about 300 K from the low-temperature end 294 toward the high-temperature end 292.

Here, a gas piston is formed in the second pulse tube 290 by appropriately controlling the open and closed states of the above-described opening and closing valves V1 to V6. The second pulse tube 290 is divided into three regions including a low-temperature region that is located on a low-temperature side of the gas piston, a high-temperature region that is located on a high-temperature side of the gas piston, and a gas piston region where the gas piston is present, due to the gas piston. A cooling stage (not shown) attached to the low-temperature end of the second pulse tube 290 is cooled by the refrigerant gas mainly present at the low-temperature region expanding.

A portion of the helium gas that flows into the low-temperature end 294 of the second pulse tube 290 stays at a low-temperature region and contributes to coldness. The remaining helium gas flows from the low-temperature region into the gas piston region, and maintains the gas piston. Hence, the refrigeration capacity of the refrigerator can be improved by reducing the amount of gas that flows from the low-temperature region into the gas piston region.

The mass of helium gas that is present in the low-temperature region is defined as M_(e). Additionally, the mass per unit time of the helium gas that flows from the low-temperature end into the low-temperature region is defined as m_(in), and the mass per unit time of the helium gas that flows out from the low-temperature region into the gas piston region is defined as m_(out). If helium gas flows into the low-temperature region, the mass M_(e) of helium gas that is present in the low-temperature region increases. On the other hand, if helium gas flows out from the low-temperature region, the mass M_(e) of helium gas that is present in the low-temperature region decreases. Accordingly, the variation dM_(e)/dt per unit time of the mass M_(e) of helium gas that is present in the low-temperature region can be expressed by a difference between the inflow mass m_(in) and the outflow mass m_(out). From the above, the following Relational Expression (1) is obtained.

m _(in) =m _(out) +dM _(e) /dt  (1)

Here, dM_(e)/dt represents the derivative of the mass M_(e) of helium gas, which is present in the low-temperature region, with respect to the time t.

Similarly, the mass of the helium gas that is present in the gas piston region is defined as Mp. Additionally, if the mass per unit time of the helium gas that flows from a gas piston region into the high-temperature region is defined as m_(h), the outflow mass can be expressed by the following Expression (2).

m _(out) =m _(h) +dM _(p) /dt  (2)

If Expression (2) is substituted into Expression (1), the following Expression (3) is obtained.

m _(in) =m _(h) +dM _(p) /dt+dM _(e) /dt  (3)

A change in the volume of the second pulse tube 290 in the low-temperature region is negligible. Thus, the volume of the second pulse tube 290 in the low-temperature region is regarded as being constant, and the value thereof is defined as Ve. Additionally, if the mean density of the helium gas in the low-temperature region is defined as ρ_(e), the mass M_(e) of the refrigerant gas that is present in the low-temperature region can be expressed by the following Expression (4).

M_(e)=V_(e)ρ_(e)  (4)

Similarly, a change in the volume of the second pulse tubes 290 in the gas piston region is negligible. Thus, the volume of the second pulse tube 290 in the gas piston region is regarded as being constant, and the value thereof is defined as V_(p). Additionally, if the mean density of the helium gas in the gas piston region is defined ρ_(p), the mass M_(p) of the refrigerant gas that is present in the gas piston region can be expressed by the following Expression (5).

M_(p)=V_(p)ρ_(p)  (5)

If Expression (4) and Expression (5) are substituted into Expression (3), the following Expression (6) is obtained.

m _(in) =m _(h) +V _(p) dρ _(p) /dt+V _(e) dρ _(e) /dt  (6)

Here, dρ_(p)/dt represents the derivative with respect to time of the density ρ_(p) of the helium gas in the gas piston region. Additionally, dρ_(e)/dt represents the derivative with respect to time of the density ρ_(e) of the helium gas in the low-temperature region.

In Expression (6), assuming that the densities of the helium gases in the low-temperature region and the gas piston region do not change with time, m_(in)=m_(h) is established, and the masses of the helium gases that are present in the low-temperature region and the gas piston region do not change. That is, this means that helium gas flows out of the gas piston region according to the amount of helium gas that has flowed into the low-temperature region. In an actual refrigeration cycle, a high-pressure helium gas is supplied to the second pulse tube 290 in the second process and the third process. As a result, a low-pressure helium gas with which the low-temperature region is filled is raised in pressure and is turned into a high-pressure helium gas.

As illustrated in FIG. 3, the high-pressure helium gas and the low-pressure helium gas have a difference in density. Accordingly, if the high-pressure helium gas flows into the low-temperature region and the low-pressure helium gas in the low-temperature region is raised in pressure and is turned into a high-pressure helium gas, a second item and a third item of a right side in Expression (6) are positive values. More specifically, the right side in Expression (6) is a density difference illustrated by a solid line in FIG. 3. From the above, the following Inequality (7) is obtained.

m _(in) −m _(h) =V _(p) dρ _(p) /dt+V _(e) dρ _(e) /dt>0  (7)

The above Inequality (7) shows that the mass of the helium gas that flows out of the gas piston region is smaller than the mass of the helium gas that flows into the low-temperature region. This allows the low-temperature region and the gas piston region to act as a so-called helium gas buffer. As a result, the pressure difference of the entire pulse tube refrigerator 200 also becomes small.

If the sixth opening and closing valve V6 is opened in the fifth process, the high-pressure helium gas within the second pulse tube 290 is turned into a low-pressure helium gas. In this case, the right side in Expression (6) has a negative value having the density difference illustrated by the solid line in FIG. 3 as an absolute value. Accordingly, the following Inequality (8) is obtained.

m _(in) −m _(out) =V _(p) dρ _(p) /dt+V _(e) dρ _(e) /dt<0  (8)

This shows that the mass of the helium gas that flows out of the gas piston region is greater than the mass of the helium gas that flows into the low-temperature region. Since a large amount of helium gas flows out due to the refrigerant flow path on the low-pressure side (suction side) of the compressor 212, a pressure drop is suppressed, and consequently, the pressure difference of the entire pulse tube refrigerator 200 becomes small.

As illustrated in FIG. 1, the second flow path resistor 261 that is a phase adjusting mechanism of the refrigerant gas is provided on the high-temperature end 292 side of the second pulse tube 290. For this reason, if helium gas is used as the refrigerant gas, the phase adjustment of the flow velocity and pressure fluctuation of the refrigerant gas on the low-temperature end 294 of the secondpulse tube 290 becomes difficult. The density difference between the high-pressure refrigerant gas and the low-pressure refrigerant gas becomes large in a region where the temperature of the refrigerant gas reaches 8 K to 30 K, and consequently, the flow rate and input work of the refrigerant gas that flows into the second pulse tube 290 become large.

In Expression (6), if the second item (V_(p)dρ_(p)/dt) of the right side can be made small and the third item (V_(e)dρ_(e)/dt) of the right side can be made correspondingly large, the mass of the helium gas of the low-temperature region that contributes to coldness can be increased without changing m_(in) and m_(h). As a result, the refrigeration performance of the refrigerator can be improved. Moreover, the amount of the helium gas of the gas piston region where the density difference between the high-pressure refrigerant gas and the low-pressure refrigerant gas becomes large can be reduced.

The pulse tube refrigerator related to an embodiment is also the same as the above-described general pulse tube refrigerator 200 in terms of basic configuration. Therefore, the pulse tube refrigerator related to the embodiment is also referred to as “a pulse tube refrigerator 200” for convenience. However, the pulse tube refrigerator 200 related to the embodiment is different from the above-described general pulse tube refrigerator 200 in terms of the configuration of the pulse tube (a second pulse tube 290 in the example illustrated in FIG. 1) having a low-temperature end at a lowest temperature, in order to make the second item (V_(p)dρ_(p)/dt) of the right side in the above-described Expression (6) small and make the third item (V_(e)dρ_(e)/dt) of the right side correspondingly large. Hereinafter, the pulse tube related to the embodiment will be described.

FIGS. 4A and 4B are views schematically illustrating the configuration of the second pulse tube 290 related to the embodiment . As illustrated in FIGS. 4A and 4B, the second pulse tube 290 related to the embodiment includes a narrowed portion 293 where a portion thereof becomes smaller than other portions in terms of the refrigerant gas flow path cross-sectional area.

In FIG. 4A, a portion of the second pulse tube 290 is narrowed to constitute the narrowed portion 293. Although not limited, the refrigerant gas flow path cross-sectional area in the narrowed portion 293 is about 30% to about 70%, more specifically, about 60% as compared to the flow path cross-sectional area of the other portions. The narrowed portion 293 is provided in a region where the temperature of the refrigerant gas reaches about 8 K to about 30 K in the second pulse tube 290, and is equivalent to the above-described gas piston region. Accordingly, when helium gas is used as the refrigerant gas, the flow path cross-sectional area of the portion of the second pulse tube 290 where the density difference between the helium gas at 2.2 MPa and the helium gas at 0.8 MPa becomes large becomes small. Namely, since the flow rate of the refrigerant gas that flows through the region of the second pulse tube 290 where the temperature of the refrigerant gas reaches about 8 K to about 30 K is decreased, a decrease in the pressure difference of the refrigerant gas in the pulse tube refrigerator 200 is suppressed, and the phase adjustment of the pressure fluctuation of the refrigerant gas becomes suitable. For this reason, the refrigeration capacity and refrigeration efficiency of the entire pulse tube refrigerator 200 can be improved.

Here, the volume of the narrowed portion 293 is defined as V_(c), and the mass of the helium gas in the narrowed portion 293 is defined as M_(c). Since the narrowed portion 293 is equivalent to the above-described gas piston region, the volume V_(c) of the narrowed portion 293 becomes smaller than the volume V_(p) of the gas piston region in Expression (6). Meanwhile, the temperature distribution of the helium gas in the above-described gas piston region is the same as the temperature distribution of the helium gas in the narrowed portion 293. For this reason, the mean density of the helium gas in the narrowed portion 293 is ρ_(p). From the above, the following Relational Expression (9) is obtained.

M _(c) =V _(c)ρ_(p) <M _(p)  (9)

The following Expression (10) is obtained from Expression (9) .

|dM _(c) /dt|<|dM _(p) /dt|  (10)

Expression (10) shows that the absolute value of a change in mass of the helium gas in the narrowed portion 293 is smaller than the absolute value of a change in mass of the helium gas in the gas piston region before providing the narrowed portion 293. A buffer action can be reduced by making the gas piston region where the contribution to the buffer action of the helium gas is large narrow. Additionally, since the third item (V_(e)dρ_(e)/dt=dM_(e)/dt) of the right side of Expression (6) can be made larger corresponding to by how much |dM_(p)/dt| is made smaller, the mass of the helium gas of the low-temperature region that contributes to coldness can be increased. As a result, the refrigeration capacity of the refrigerator can be improved.

In the case of a two-stage type pulse tube refrigerator as illustrated in FIG. 4A, the first regenerator 240 on the high-temperature side and the second regenerator 280 on the low-temperature side are arranged so as to share a central axis. The second pulse tube 290 is arranged so as to become substantially parallel to the first regenerator 240 and the second regenerator 280. Additionally, a straight line connecting the low-temperature end 294 of the second pulse tube 290 and the low-temperature end 284 of the second regenerator 280 is substantially orthogonal to the central axis of the second pulse tube 290 and the central axis of the second regenerator 280. That is, when the second pulse tube 290, the first regenerator 240, and the second regenerator 280 are installed so that the longitudinal directions thereof become a vertical direction, the low-temperature end 294 of the second pulse tube 290 and the low-temperature end 284 of the second regenerator 280 have a substantially equal height.

For convenience of description, as illustrated in FIG. 4A, an x-coordinate axis is set in the longitudinal direction of the second pulse tube 290 from the low-temperature end 294 of the second pulse tube 290 toward the high-temperature end 292, with the low-temperature end 294 of the second pulse tube 290 as an origin. Since the first regenerator 240 and the second regenerator 280 are arranged side by side, it is possible to match an x-coordinate in the second pulse tube 290 with x-coordinates in the first regenerator 240 and the second regenerator 280. Accordingly, in the following present specification, for example, the position of the second pulse tube 290 corresponding to the second regenerator 280 means the position of the second pulse tube 290 that has the same coordinate as the x-coordinate of the second regenerator 280. The position in the second pulse tube 290 corresponding to the low-temperature end 284 of the second regenerator is the low-temperature end 294 of the second pulse tube 290.

Here, the temperature of the refrigerant gas at the high-temperature end 242 of the first regenerator 240 is about a room temperature, and the temperature of the refrigerant gas at the low-temperature end 244 of the first regenerator 240 is about 50 K. The temperature of the refrigerant gas at the high-temperature end 244 of the second regenerator 280 is about 50 K, and the temperature of the refrigerant gas at the low-temperature end 284 of the second regenerator 280 is about 4K. The temperature of the refrigerant gas at the low-temperature end 294 of the second regenerator is also about 4 K, and the temperature of the refrigerant gas at the high-temperature end 292 reaches about a room temperature. The temperature of the refrigerant gas at a predetermined position in the first regenerator 240 or the second regenerator 280 and the temperature of the refrigerant gas at a corresponding position in the second pulse tube 290 become substantially equal. Accordingly, the temperature of the refrigerant gas of the second pulse tube 290 at a position corresponding to the high-temperature end 244 of the second regenerator 280 reaches about 50K.

As illustrated in FIG. 3, when the temperature is 50 K or lower the density difference between the helium gas at 2.2 MPa and the helium gas at 0.8 MPa becomes larger as the temperature decreases until the temperature reaches about 8 K. Thus, the narrowed portion 293 in the second pulse tube 290 is provided nearer the low-temperature end 294 side than the position corresponding to the high-temperature end 244 of the second regenerator 280.

The first regenerator 240 and the second regenerator 280 related to the embodiment include a regenerator material, respectively. The second regenerator 280 includes a first regenerator material 281 arranged on the high-temperature side, and a second regenerator material 283 arranged on the low-temperature side as two types of regenerator material, and the first regenerator material 281 and the second regenerator material are adjacent to each other. It is preferable that the narrowed portion 293 in the second pulse tube 290 be provided nearer the high-temperature end 292 side than a position corresponding to the boundary between the first regenerator material 281 and the second regenerator material 283 in the second regenerator 280. Furthermore, it is preferable that the temperature of the refrigerant gas on the low temperature side of the narrowed portion 293 be about 8 K. Accordingly, the narrowed portion 293 is provided in the region of the second pulse tube 290 where the temperature of the refrigerant gas reaches about 8 K to about 30 K.

In addition, the narrowed portion 293 in the second pulse tube 290 is about 60% of the flow path cross-sectional area of the other portion. Therefore, if the flow path cross-sectional area changes rapidly at a boundary portion between the narrowed portion 293 and another region, turbulence may occur in the refrigerant gas, and pressure loss may occur. Therefore, it is preferable that the narrowed portion 293 have a taper shape in which the flow path cross-sectional area changes gradually at the boundary portion with the other region.

More specifically, as illustrated in FIG. 4A, the narrowed portion 293 increases gradually at the boundary portion on a high-temperature end side with the other region until the flow path cross-sectional area thereof reaches the flow path cross-sectional area in the other region. Similarly, the narrowed portion 293 increases gradually at the boundary portion on a low-temperature end side with the other region until the flow path cross-sectional area thereof reaches the flow path cross-sectional area in the other region.

FIG. 4B is a view illustrating another configuration of the narrowed portion 293 in the second pulse tube 290 . The narrowed portion 293 illustrated in FIG. 4A is configured by narrowing the second pulse tube 290. In contrast, in the example illustrated in FIG. 4B, the external diameter of the second pulse tube 290 in the narrowed portion 293 is not different from the external diameter of the other portion. Instead, the flow path cross-sectional area is made smaller by inserting a filling member into the second pulse tube 290 . The filling member can be realized by appropriately using metal, resin, plastic, or the like.

The example illustrated in FIG. 4B also has the same effects as obtained using the narrowed portion 293 itself as in the example illustrated in FIG. 4A. The example illustrated in FIG. 4B has an advantage in that the strength of the narrowed portion 293 in the second pulse tube 290 is improved, as compared to the example illustrated in FIG. 4A.

As described above, the pulse tube refrigerator 200 related to the embodiment includes the second pulse tube 290 that has the narrowed portion 293 provided in a portion thereof. Accordingly, the flow rate of the refrigerant gas of the second pulse tube 290 can be reduced, a decrease in the pressure difference of the refrigerant gas in the pulse tube refrigerator 200 can be suppressed, and the phase between the flow velocity and the pressure fluctuation of the refrigerant gas can be optimized. As a result, the refrigeration capacity and refrigeration efficiency of the entire pulse tube refrigerator 200 can be improved.

Although the invention has been described above on the basis of the embodiment, the embodiment is merely illustrative of the principle and application of the invention. Additionally, various modification examples and various changes of arrangement can be made to the embodiment, without departing from the idea of the invention defined in the scope of the claims.

Modification Example

The pulse tube refrigerator 200 has been described above setting the two-stage type structure as a premise. The pulse tube refrigerator 200 is not limited to the two-stage type, and the invention can be applied to a multistage type. Hereinafter, a pulse tube refrigerator 201 having a three-stage type structure will be described as an example of a multistage type.

FIG. 5 is a view schematically illustrating the outline of an example of the four-valve type pulse tube refrigerator 201 related to a modification example of the invention. The pulse tube refrigerator 201 has a three-stage type structure. In addition, in FIG. 5, the same reference numerals as those of FIG. 1 are given to members the same as those of the aforementioned FIG. 1.

The three-stage type pulse tube refrigerator 201 has the same configuration as the aforementioned two-stage type pulse tube refrigerator 200. Here, the three-stage type pulse tube refrigerator 201 further has a third regenerator 440 and a third pulse tube 420.

The third regenerator 440 has a high-temperature end 284 (equivalent to the low-temperature end 284 of the second regenerator 280), and a low-temperature end 444. The third pulse tube 420 has a high-temperature end 422 and a low-temperature end 424, and heat exchangers are installed at the high-temperature end 422 and the low-temperature end 424. The low-temperature end 444 of the third regenerator 440 is connected to the low-temperature end 424 of the third pulse tube 420 via third piping 416.

The refrigerant flow path on the high-pressure side (discharge side) of the compressor 212 has a fourth refrigerant supply path H4, in addition to the first refrigerant supply path H1, the second refrigerant supplypathH2, andthethirdrefrigerant supply path H3 as illustrated in FIG. 1. Additionally, the refrigerant flow path on the low-pressure side (suction side) of the compressor 212 has a fourth refrigerant recovery path L4, in addition to the first refrigerant recovery path L1, the second refrigerant recovery path L2, and the third refrigerant recovery path L3 as illustrated in FIG. 1.

The fourth refrigerant supply path H4 is constituted of the high-pressure side of the compressor 212, fourth high-pressure-side piping 245A to which a seventh opening and closing valve V7 is connected, common piping 455 where a flow path resistor 450 is installed, and the third pulse tube 420. The fourth refrigerant recovery path L4 is constituted of the third pulse tube 420, the common piping 455 where the flow path resistor 450 is installed, fourth low-pressure-side piping 245B where an eighth opening and closing valve V8 is installed, the point B, and the path of the compressor 212 . The flow path resistor 450 is constituted of an orifice or the like.

Next, the operation of the four-valve type pulse tube refrigerator 201 configured in this way will be described.

FIG. 6 is a view illustrating a series of open/closed states of eight opening and closing valves V1 to V8 in the operation of the four-valve type pulse tube refrigerator 201 illustrated in FIG. 5. Hereinafter, the view illustrated in FIG. 5 is also referred to as a “second timing chart”. During the operation of the pulse tube refrigerator 201, the open/closed states of the eight opening and closing valves V1 to V8 change periodically as follows.

(First Process: Time 0 to Time t3)

First, at time t=0, only the seventh opening and closing valve V7 is open. Accordingly, a high-pressure refrigerant gas is supplied from the compressor 212 via the fourth refrigerant supply path H4, that is, via the path of the fourth high-pressure-side piping 245A, the common piping 455, and the high-temperature end 422 to the third pulse tube 420. Thereafter, at time t=t1, the fifth opening and closing valve V5 is opened while the seventh opening and closing valve V7 remains open. Accordingly, the high-pressure refrigerant gas is supplied from the compressor 212 via the third refrigerant supply path H3, that is, via the path of the third high-pressure piping 235A, the common piping 299, and the high-temperature end 292 to the second pulse tube 290.

Next, at time t=t2, the third opening and closing valve V3 is opened in a state where the opening and closing valves V7 and V5 are open. Accordingly, the high-pressure refrigerant gas is supplied from the compressor 212 via the second refrigerant supply path H2, that is, via the path of the second high-pressure piping 225A, the common piping 230, and the high-temperature end 252 to the first pulse tube 250.

Next, at time t=t3, the first opening and closing valve V1 is opened in a state where the opening and closing valves V7, V5, and V3 are open. Accordingly, the high-pressure refrigerant gas is introduced into the first regenerator 240, the second regenerator 280, and the third regenerator 440. A portion of the refrigerant gas flows from the low-temperature end 254 side via the first piping 256 into the first pulse tube 250. Additionally, another portion of the refrigerant gas passes through the second regenerator 280, and flows from the low-temperature end 294 side via the second piping 286 into the second pulse tube 290. Still another portion of the refrigerant gas passes through the third regenerator 440, and flows from the low-temperature end 424 side via the third piping 416 into the third pulse tube 420.

(Second Process: Time t4 to Time t7)

Next, at time t=t4, the third opening and closing valve V3 is closed while the opening and closing valves V1, V5, and V7 remain open. Thereafter, the fifth and seventh opening and closing valves V5 and V7 are also closed sequentially (time t=t5 and t=t6). Correspondingly, the refrigerant gas from the compressor 212 flows into the first regenerator 240 only via the first refrigerant supply path H1. Thereafter, the refrigerant gas flows from the respective low-temperature end 254, 294, and 424 sides into the first pulse tube 250, the second pulse tube 290, and the third pulse tube 420.

At time t=t7, all the opening and closing valves V1 to V8 are closed. Due to pressure rises in the first pulse tube 250, the second pulse tube 290, and the third pulse tube 420, the refrigerant gas within the first pulse tube 250, the second pulse tube 290, and the third pulse tube 420 moves toward reservoirs (not illustrated) installed on the respective high-temperature end 252, 292, and 422 sides.

(Third Process: Time t7 to Time t10)

Thereafter, at time t=t7, the eighth opening and closing valve V8 is opened, and the refrigerant gas within the third pulse tube 420 returns to the compressor 212 through the fourth refrigerant recovery path L4. Thereafter, at time t=t8, the sixth opening and closing valve V6 is opened, and the refrigerant gas within the second pulse tube 290 returns to the compressor 212 through the third refrigerant recovery path L3. Accordingly, the pressure of the third pulse tube 420 and the pressure of the second pulse tube 290 drop. Thereafter, at time t=t9, the fourth opening and closing valve V4 is opened, and the refrigerant gas within the first pulse tube 250 returns to the compressor 212 through the second refrigerant recovery path L2. Accordingly, the pressure of the first pulse tube 250 drops.

Moreover, at time t=t10, the second opening and closing valve V2 is opened while the opening and closing valves V8, V6, and V4 remain open. Accordingly, a major portion of the refrigerant gas within the third pulse tube 420, the second pulse tube 290, the first pulse tube 250, and the first regenerator 240, the second regenerator 280, and the third regenerator 440 passes through the first regenerator 240, and returns to the compressor 212 via the first refrigerant recovery path L1.

(Fourth Process: Time t11 to Time t14)

Next, at time t=t11, the fourth opening and closing valve V4 is closed in a state after opening of the opening and closing valves V2, V6, and V8, and thereafter, the sixth opening and closing valve V6 and the eighth opening and closing valve V8 are sequentially closed (time t=t12 and t=t13).

Finally, at time t=t14, the second opening and closing valve V2 is closed, and one cycle is completed.

By repeating the above cycle, coldness occurs at the low-temperature end 254 of the first pulse tube 250, the low-temperature end 294 of the second pulse tube 290, and the low-temperature end 424 of the third pulse tube 420, and a cooling target can be cooled.

In the pulse tube refrigerator 201 related to the modification example, the temperature of the refrigerant gas that flows through the low-temperature end 444 of the third regenerator 440 reaches about 4 K. For this reason, the temperature of the refrigerant gas that flows through the low-temperature end 424 of the third pulse tube 420 also reaches about 4 K. The refrigerant gas within the third pulse tube 420 is at about a room temperature at the high-temperature end 422 of the third pulse tube 420.

In the pulse tube refrigerator 201, the first regenerator 240, the second regenerator 280, and the third regenerator 440 are arranged so as to share a longitudinal central axis, and the third pulse tube 420 and the third regenerator 440 are arranged side by side. Accordingly, a position corresponding to a position in the third regenerator 440 can be fixed in the third pulse tube 420, similar to the relationship between the second pulse tube 290 and the second regenerator 280 illustrated in FIG. 1.

In the pulse tube refrigerator 201 related to the modification example, a narrowed portion 493 where the refrigerant gas flow path cross-sectional area becomes smaller is provided in a region where the temperature of the refrigerant gas that flows through the third pulse tube 420 reaches about 8 K to 20 K. The narrowed portion 493 in the third pulse tube 420 is located nearer the low-temperature side than a position corresponding to the high-temperature end 284 of the third regenerator 440 that is a regenerator at a last stage.

Accordingly, the flow rate of the region which at the density difference between the helium gas at 2.2 MPa and the helium gas at 0.8 MPa becomes large can be reduced, and a decrease in the pressure difference of the refrigerant gas in the pulse tube refrigerator 200 can be suppressed. Additionally, the phase adjustment of the pressure fluctuation of the refrigerant gas can also be optimized. As a result, the refrigeration capacity and refrigeration efficiency of the entire pulse tube refrigerator 200 can be improved.

In addition, similar to the two-stage type pulse tube refrigerator 200 and the three-stage type pulse tube refrigerator 201, even in refrigerators having a number of stages of equal to or more than two or three stages, the same effects can be achieved by providing the narrowed portion in a portion of a pulse tube at the last stage on the low-temperature side.

It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention. 

What is claimed is:
 1. A pulse tube refrigerator comprising: a compressor that compresses a refrigerant gas; a first pulse tube that has a low-temperature end and a high-temperature end connected to the compressor; a first regenerator that has a low-temperature end connected to the low-temperature end of the first pulse tube and a high-temperature end connected to the compressor; a second pulse tube that has a high-temperature end connected to the compressor and a low-temperature end having a lower temperature than the low-temperature end of the first pulse tube; and a second regenerator that has a high-temperature end and a low-temperature end connected to the low-temperature end of the second pulse tube and is arranged side by side with the second pulse tube, wherein the second pulse tube includes a narrowed portion nearer a low-temperature end side than a position corresponding to the high-temperature end of the second regenerator.
 2. The pulse tube refrigerator according to claim 1, wherein the second regenerator includes: a first regenerator material that is arranged on a high-temperature side; and a second regenerator material that is arranged on a low-temperature side and is adjacent to the first regenerator material, and wherein the narrowed portion is provided nearer the low-temperature end side than a position corresponding to a boundary between the first regenerator material and the second regenerator material in the second regenerator.
 3. The pulse tube refrigerator according to claim 1, wherein the narrowed portion is provided in a region where the temperature of the refrigerant gas flowing through the second pulse tube reaches 8 K to 30 K.
 4. The pulse tube refrigerator according to claim 1, wherein the narrowed portion increases gradually at a boundary portion on a high-temperature end side with another region until the flow path cross-sectional area thereof reaches a flow path cross-sectional area in the other region.
 5. The pulse tube refrigerator according to claim 1, wherein the narrowed portion increases gradually at a boundary portion on a low-temperature end side with another region until the flow path cross-sectional area thereof reaches a flow path cross-sectional area in the other region.
 6. The pulse tube refrigerator according to claim 1, wherein the second pulse tube is a pulse tube at a last stage on a low-temperature side, and wherein the second regenerator is a regenerator at the last stage on the low-temperature side.
 7. A pulse tube refrigerator comprising: a compressor that compresses a refrigerant gas; a first pulse tube that has a low-temperature end and a high-temperature end connected to the compressor; a first regenerator that has a low-temperature end connected to the low-temperature end of the first pulse tube and a high-temperature end connected to the compressor; a second pulse tube that has a high-temperature end connected to the compressor and a low-temperature end having a lower temperature than the low-temperature end of the first pulse tube; and a second regenerator that has a high-temperature end and a low-temperature end connected to the low-temperature end of the second pulse tube and is arranged side by side with the second pulse tube, wherein the second pulse tube includes a narrowed portion in a region where the temperature of the refrigerant gas flowing through the second pulse tube reaches 8 K to 30 K. 